This proposal presents a program of spectroscopic studies of porphyrin model complexes with magnetic circular dichroism (MCD) spectroscopy and of spectroscopic and mechanistic studies of four oxygen and peroxide metabolizing heme iron proteins: cytochrome P-450, secondary amine mono-oxygenase (SAMO), indoleamine dioxygenase (IDO) and chloroperoxidase (CPO). There are three overall objectives. (1) By investigating model porphyrin and heme protein ligand complexes of defined structure, the utility of MCD spectroscopy as a probe of heme iron electronic and, ultimately, physical structure will be ascertained. We will examine synthetic Fe+2/+3(porph)(RS-)(X)and Fe+2(porph)(X)(CO) complexes with variable biomimetic ligands (X), Fe +2/+3(porph)imidazolate complexes and unusual six-coordinate high-spin FE+3 porphyrins in order to establish spectroscopic fingerprints for each ligand and/or complex type to use in structure determination. We will also examine zinc porphyrin complexes where axial ligation effects can be more directly assessed. Next we will investigate IDO and CPO and their ligand complexes so as to better define their coordination structures. (2) Studies designed to define the structural requirements for heme-iron catalyzed hydroxylation of organic substrates by further characterization of the active site structures of P-450 and SAMO will be carried out. Studies from our laboratory and elsewhere indicate that they have different metal ion environments; our work suggests subtle distinctions between SAMO and myoglobin as well. Spectroscopic studies designed to better define the ligand environment of SAMO will continue. The sixth ligand to ferric P-450 may be an oxygen donor; to look directly for a water ligand, we will examine the EPR spectrum of P-450 in 17O H2O. (3) We will test current hypotheses about the mechanism of N-dealkylation by P-450 and SAMO by doing direct 18O labelling experiments with substrates that form stable carbinolamine and related products rather than dealkylating. The stable product can then be tested for label incorporation from either dioxygen or water. New substrates for SAMO will be sought out; its interaction with substrates, electrons and oxygen will be studied. The ubiquitous involvement of oxygen in the metabolism of steroids, drugs and other molecules and of P-450 in carcinogenesis increase the importance of the fundamental studies of the structure and mechanism of action of heme iron oxygenases described herein.